1. Field of the Invention
This invention relates to an automatic transmit power control method and system for radio equipment, and more particularly to an automatic transmit power control method and system for radio equipment of a cross polarization interference canceler system (XPIC system).
2. Description of the Related Art
It is common practice in radio communications to set different signal channels for two orthogonally polarized waves of the same frequency channel in order to make effective use of the frequency resources. The two orthogonally polarized waves may be, for example, a vertically polarized wave and a horizontally polarized wave. In the following description, the vertically polarized wave and the horizontally polarized wave are referred to as V-polarized wave and H-polarized wave, respectively. The XPIC system is one radio communications system wherein the two orthogonally polarized waves are used commonly in communications from the transmission side to the reception side while canceling the interference between the polarized waves to enhance the independence of the individual signal channels. An XPIC system is employed, for example, in a microwave digital communications network.
In microwave communications, a single transmission band is divided into a plurality of frequency channels, and a transmitter and a receiver are provided for each of the frequency channels. When the XPIC system is applied to microwave communications, a transmitter and a receiver are provided for each V-polarized wave and each H-polarized wave of each of the frequency channels.
In microwave communications, only one particular frequency channel sometimes suffers from a high transmission loss due to multipath fading or from some other cause. When there is the influence of deep fading, not only the S/N ratio of the particular frequency channel is deteriorated, but also interference from an adjacent frequency channel or channels is increased, resulting in deterioration in the quality of received signals. Therefore, an ATPC (Automatic Transmit Power Control) system has been developed which increases the transmit power only for a particular frequency channel which undergoes deep fading. The ATPC system is generally so constructed that, when a drop of the receive signal level or reception intensity on the reception side is detected, the information is transmitted to the transmission side by way of a control circuit or control line in the opposite direction, and on the transmission side, the transmitter is controlled so as to increase the transmit power. Naturally, when the influence of multipath fading disappears, the original transmit power is restored.
Here, an application of an ATPC system to an XPIC system is investigated. Since control is effected for each pair of transmitters and receivers in a conventional ATPC system, ATPC control is effected for the V-polarized wave and the H-polarized wave of the same frequency channel independently of each other. FIG. 1 shows an exemplary conventional ATPC system applied to an XPIC system. Only the construction of a conventional ATPC system for a frequency channel whose center frequency is f.sub.1 is shown in FIG. 1.
Referring to FIG. 1, the transmission site includes transmitter 51a for a V-polarized wave, circulator 52a provided on the output side of transmitter 51a, automatic transmit power controller 53a for controlling the output power of a V-polarized wave transmitter 51a, transmitter 51b for a H-polarized wave, circulator 52b provided on the output side of transmitter 51b, automatic transmit power controller 53b for controlling the output power of transmitter 51b, and antenna 54 provided commonly for the V-polarized wave and the H-polarized wave. Each of circulators 52a and 52b is connected to an individual receiver (not shown) which is provided for transmission in the opposite direction. Antenna 54 is so constructed that it can radiate a V-polarized wave and a H-polarized wave simultaneously without mixing them.
Meanwhile, the reception site includes receiver 57a for a V-polarized wave, circulator 56a provided on the input side of receiver 57a, controller 58a for controlling automatic transmit power controller 53a of the transmission site in response to the reception intensity of the V-polarized wave, receiver 57b for a H-polarized wave, circulator 56b provided on the input side of receiver 57b, controller 58b for controlling automatic transmit power controller 53b of the transmission site in response to the reception intensity of the H-polarized wave, and antenna 55 provided commonly for the V-polarized wave and the H-polarized wave. Antenna 55 is capable of receiving a V-polarized wave and a H-polarized wave simultaneously and separating them from each other. V-polarized wave components of the received wave are supplied to circulator 56a while H-polarized wave components of the received wave are supplied to circulator 56b. Each of circulators 56a and 56b is connected to an individual transmitter (not shown) which is provided for transmission in the opposite direction. Receivers 57a and 57b have an identical construction and each includes first level detector 61 for detecting the reception intensity of a frequency channel whose center frequency is f.sub.1, and second level detector 62 for detecting the reception intensity of an adjacent frequency channel. Second level detector 62 includes band eliminating filter 63 and detection element 64. An input signal to receiver 57a or 57b is supplied to first and second level detectors 61 and 62.
Controller 58a and automatic transmit power controller 53a for the V-polarized wave are interconnected by way of control line 59a which extends from the reception site to the transmission site. Controller 58a sends out an automatic transmit power control signal to automatic transmit power controller 53a in response to the outputs of first and second level detectors 61 and 62 of receiver 57a. The automatic transmit power control signal is used to increase the transmit power of transmitter 51a for a V-polarized wave when the reception sensitivity for the object frequency channel (whose center frequency is f.sub.1) drops to a level lower than a certain threshold level or when the difference between the level of the object frequency channel and the level of an adjacent frequency channel exceeds another certain threshold level. Similarly, controller 58b and automatic transmit power controller 53b for the H-polarized wave are interconnected by way of control line 59b, which transmits an automatic transmit power control signal for a H-polarized wave therethrough.
Automatic control of the transmit power in the ATPC system is next described with reference to FIG. 2(a), FIG. 2(b), and FIG. 2(c) The frequency configuration as shown in the waveform curve of FIG. 2(a) is assumed first. In particular, two adjacent frequency channels (whose center frequencies are f.sub.0 and f.sub.2) are present for an object frequency channel (whose center frequency is f.sub.1). Two orthogonal polarized waves are used independently of each other at least for the object frequency channel. Here, it is assumed that the reception sensitivity for the object frequency channel is dropped by multipath fading until the level difference of the V-polarized wave of the object frequency channel from that of an adjacent frequency channel exceeds a predetermined threshold level as shown by the waveform curve in FIG. 2(b). Then, an automatic transmit power control signal to increase the transmit power is transmitted from controller 58a to automatic transmit power controller 53a. Consequently, the reception sensitivity for the V-polarized wave side of the object frequency channel is raised as seen from the waveform curve of FIG. 2(c). Since the transmit power on the H-polarized wave side, however, remains without change, then an increase of the transmit power on the V-polarized wave side will result in an increase in interference from the V-polarized wave side to the H-polarized wave side, and will consequently result in deterioration of the signal of the H-polarized wave of the object frequency channel.